1. Technical Field
This invention relates generally to processes for conducting Fischer-Tropsch synthesis, and, more specifically, to processes employing a slurry bubble column reactor and catalysts therefore to carry out three phase Fischer-Tropsch reactions.
2. Background
Synthesis gas, or "syngas" is a mixture consisting primarily of hydrogen and carbon oxides. Syngas is produced during coal gasification, and processes are well known to obtain syngas from other hydrocarbons, including natural gas. U.S. Pat. No. 4,423,265 to Chu et al., Col. 1, notes that processes for producing syngas of major importance depend either on the partial combustion of the hydrocarbon fuel with an oxygen-containing gas or on the reaction of the fuel with steam, or on a combination of these two reactions. U.S. Pat. No. 5,324,335 to Benham et al., Col. 2, explains the two primary methods for producing syngas from methane: steam reforming and partial oxidation. The Encyclopedia of Chemical Technology, Second Edition, Volume 10, pages 353-433 (1966), Interscience Publishers, New York, N.Y. and Third Edition, Volume 11, pages 410-446 (1980), John Wiley and Sons, New York, N.Y. is said by Chu et al. to contain an excellent summary of gas manufacture, including the manufacture of synthesis gas.
It has long been recognized that syngas can be converted to liquid hydrocarbons by the catalytic hydrogenation of carbon monoxide. The general chemistry of the much studied Fischer-Tropsch synthesis is as follows: EQU (1) CO+2H.sub.2 .fwdarw.(--CH.sub.2 --)+H.sub.2 O EQU (2) 2CO+H.sub.2 .fwdarw.(--CH.sub.2 --)+CO.sub.2
The types and amounts of reaction products, i.e. the length of carbon chains, obtained via Fischer-Tropsch synthesis varies dependent upon process kinetics and choice of catalyst.
Many attempts at providing effective catalysts for selectively converting syngas to liquid hydrocarbons have previously been disclosed. U.S. Pat. No. 5,248,701 to Soled et al., Col. 1-3, presents an overview of relevant patent art.
The two most popular types of catalysts heretofore used in the Fischer-Tropsch synthesis are iron-based catalysts and cobalt-based catalysts. U.S. Pat. No. 5,324,335 to Benham et al., Col. 1, discusses the fact that iron-based catalysts, due to their high water gas shift activity, favor the overall reaction shown in (2) above, while cobalt-based catalysts tend to favor the overall reaction of scheme (1).
Recent advances have provided a number of catalysts active in Fischer-Tropsch synthesis. Besides iron and cobalt, other Group VIII metals, particularly ruthenium, are known Fischer-Tropsch catalysts.
It is the current practice to support such catalysts on a porous inorganic refractory oxide. Particularly preferred supports include silica, alumina, silica-alumina, and titania. In addition, other refractory oxides selected from Groups III, IV, V, VI and VIII may be used as a catalyst support.
The prevailing practice is also to add promoters to the supported catalyst. Promoters can include ruthenium (when not used as the primary catalyst), rhenium, hafnium, cerium and zirconium. Promoters are known to increase the activity of the catalyst, sometimes rendering the catalyst three to four times as active as its unpromoted counterpart.
Contemporary cobalt catalysts are typically prepared by impregnation of the catalyst upon the support. As described in U.S. Pat. No. 5,252,613 to Chang et al., Col. 4-5, a typical catalyst preparation may involve impregnation, by incipient wetness or other known techniques, of, for example, a cobalt nitrate salt onto a titania, silica or alumina support, optionally followed or preceded by impregnation with a promotor material. Excess liquid is removed and the catalyst precursor is dried. Following drying or as a continuation thereof, the catalyst is calcined to convert the salt or compound to its corresponding oxide(s). The oxide is then reduced by treatment with hydrogen or a hydrogen containing gas for a period of time sufficient to substantially reduce the oxide to the elemental or catalytic form of the metal. U.S. Pat. No. 5,498,638 to Long points to U.S. Pat. Nos. 4,637,993, 4,717,702, 4,477,595, 4,663,305, 4,822,824, 5,036,032, 5,140,050, 5,292,705 as disclosing well known catalyst preparation techniques.
Fischer-Tropsch synthesis has primarily been conducted in fixed bed reactors, gas-solid reactors, and gas entrained fluidized bed reactors, fixed bed reactors being the most utilized. U.S. Pat. No. 4,670,472 to Dyer et al., Col. 1, provides a bibliography of several references describing these systems.
More recently, however, attention has been directed to conducting Fischer-Tropsch synthesis in three phase slurry reactors. Three phase reactions involve the introduction of a fluidizing gas into a reactor containing catalyst particles slurried in a liquid. Particularly useful in Fischer-Tropsch processes is the slurry bubble column reactor (SBCR). In an SBCR, catalyst particles are slurried in liquid hydrocarbons within a reactor chamber, typically a tall column. Syngas is then introduced at the bottom of the column through a distributor plate, which produces small gas bubbles. The gas bubbles migrate up and through the column, causing a beneficial turbulence, while reacting with the catalyst to produce liquid and gaseous hydrocarbon products. Gaseous products are captured at the top of the SBCR, while liquid products are recovered through a filter which separates the liquid hydrocarbons from the catalyst fines. U.S. Pat. Nos. 4,684,756, 4,788,222, 5,157,054, 5,348,982, and 5,527,473 reference this type of system and provide citations to pertinent patent and literature art.
Using an SBCR to conduct Fischer-Tropsch synthesis has certain recognized advantages. As noted by Rice et al. in U.S. Pat. No. 4,788,222, Col. 5, advantages of a slurry process over that of a fixed bed process include better control of the exothermic heat produced during the reactions and better control over catalyst activity maintenance by allowing continuous recycling, recovery and rejuvenation procedures to be implemented. U.S. Pat. Nos. 5,157,054, 5,348,982, and 5,527,473 also discuss advantages flowing from the use of an SBCR. Heretofore, catalyst particle size has not been deemed to be a critical parameter in SBCRs. It is desired that the catalyst particle be reasonably filterable, but also easily dispersible. The art suggests that particle sizes of 1-200 microns meet these requirements. (See Chang, Col. 5).
Notwithstanding the research and development heretofore conducted, Fischer-Tropsch synthesis in a three phase slurry bubble column reactor is by no means a refined procedure. The process remains expensive, owing in part to the significant cost of promoted catalysts in the current state of the art. Environmental concerns also come into play, not only with respect to the operation of an SBCR, but also with regard to the preparation of catalysts, which involves the use of organic solvents. The SBCR process is also extremely demanding upon the catalyst from a physical strength standpoint which leads to severe attrition of the catalyst particles and resulting in catalyst loss or filtration problems.
The present invention encompasses certain discoveries that have resulted in a more rate efficient, environmentally friendly and more cost efficient process for conducting Fischer-Tropsch synthesis in a slurry bubble column reactor.